1. Field of the Invention
This invention relates to band printers in which a steel printing band containing type characters if formed into a loop around a pair of pulleys. The pulleys are rotated in order to move the band and present different characters to a printing medium. More particularly, this invention relates to an improved pulley for use with such band printers.
2. Description of the Prior Art
In band printers, a critical requirement is that the band be maintained in proper vertical alignment so as to achieve accurate print registration on the printing medium. One method of providing band alignment in pulley systems involves the use of crowned pulleys, as shown in FIG. 2. A crowned pulley (i.e., one in which the middle of the pulley surface extends beyond its edge) provides a self centering action on a band by causing the center of the band to align with the top of the crown. Such a system cannot successfully be used with band printers because variations in the thickness of the band (due both to inconsistencies in the band itself and the non-uniformity of etched print characters) would cause the true, or elastic, center of the band to vary, resulting in a slight amount of "floating" of the band. This floating would result in completely unacceptable performance if a system if this type were used in a band printer.
A commonly utilized prior art band printer pulley design is shown in FIG. 3. In this type of design, the pulleys which carry the band have a slanted outer surface. The printing band is looped around a pair of pulleys having parallel axes. The slanting surface of the outer shell causes the tensioned band to move downward and contact a pair of steel roller bearings 34 as it is moved around the pulleys. The roller bearings serve to maintain the band in a fixed vertical position. Although the bearings are made of steel, the motion and sharpness of the steel band causes grooves to be slowly cut into the bearings. Since the bearings establish the vertical alignment of the printing band, the gradual formation of the groove will eventually cause the band to become misaligned, resulting in misregistration during printing. The pulleys can either be maintained in a fixed vertical position or can be mounted so as to allow free vertical movement. In the latter instance, the weight of the pulley is the only downward force acting upon the band, and the force on the bearings is somewhat reduced. However, the use of such a pulley mounting does not completely eliminate the problem of groove formation.
It is accordingly an object of the present invention to provide a pulley which provides accurate long term vertical positioning of the printer band. It is a further object of the present invention to provide a pulley which is low in cost and maintains its performance despite variations in environmental conditions such as humidity.